Scientists at Washington University School of Medicine in St. Louis have identified the molecular signal that triggers the development of immune cells that patrol the skin and brain. Both the immune cells that protect the skin, called Langerhans cells, and those that protect the brain, called microglia, have specialized features that make them better defenders of their respective territories. Researchers now report in Nature Immunology that the cells only acquire those characteristics after they bind to an immune system molecule known as IL-34.